1. Field of the Invention
This invention relates to amplifier pre-distortion, and more particularly amplifier pre-distortion and efficient use of amplifier power capacity, for example, in a wireless communication system.
2. Description of Related Art
Amplifier sharing is a useful technique in many signal-processing contexts. In a typical application of this technique, several original signals are used to form composite signals of equal power. Each of the composite signals is amplified in a respective amplifier. Thus, even if one of the original signals has a much larger power than the other original signals, because the composite signals have equal power, the amplifiers can have an equal power capacity, and that power capacity can be smaller than that of a single amplifier that had to amplify the original signal with the much larger power. This allows the amplifiers to be used more efficiently than would be the case if the respective amplifiers had amplified the original signals, which reduces wasted amplifier power capacity and its associated costs.
Amplifier sharing is particularly useful in applications that support both transmitting a signal using so-called transmit diversity and transmitting a signal without using transmit diversity. Amplifier sharing allows the signal that is to be transmitted without transmit diversity to form two signals of equal power, which allows the use of equal sized amplifiers to amplify signals that are to be transmitted without using transmit diversity. This allows the amplifiers to be used more efficiently by reducing wasted amplifier power capacity. Such an application is described in more detail below and in patent application “Power Amplifier Sharing In A Wireless Communication System With Transmit Diversity”, Ser. No. 09,631,855 assigned to the same Assignee hereof. Amplifier sharing is also useful for sharing amplifiers between two so-called sectors of a cell of a wireless telecommunication system. This is because with amplifier sharing the signals at the input of each amplifier have equal power even if the power of the signals going to mobile terminals in one sector is much larger than the power of the signals going to mobile terminals in another sector.
The amplifiers that are to be shared can be in a transmitter that modulates the original signals onto RF signals. In a wireless environment this function is performed in a so-called radio. The transmitter therein forms the composite signals on the RF signals, amplifies the composite signals, and then uses the amplified composite signals to form the amplified original signals. A significant goal in prior art amplifier-sharing arrangements is to avoid having to phase and gain match the radios to produce an acceptable antenna isolation tolerance. This is readily achieved by forming the composite signals in the analog domain, i.e. using analog circuitry.
Whether or not the amplifiers are being shared, it is advantageous for them to be linear; otherwise the signal is distorted. To compensate for non-linearities that occur at high power levels, a feed-forward loop following the amplification can be used. Inasmuch as composite signals are formed in the analog domain for the reasons just stated, the feed-forward loop is also implemented in the analog domain.